Largest prime factor

Problem Description

Everybody knows any number can be combined by the prime number.
Now, your task is telling me what position of the largest prime factor.
The position of prime 2 is 1, prime 3 is 2, and prime 5 is 3, etc.
Specially, LPF(1) = 0.

Input

Each line will contain one integer n(0 < n < 1000000).

Output

Output the LPF(n).

Sample Input

Sample Output

采用素数筛选法：即先将2的倍数全置1，然后将3的倍数置2，……

View Code

 1 #include<iostream>
 2 using namespace std;
 3 #define N 1000001
 4 int prime[N];
 5 int main()
 6  {
 7       for(int j=0;j<N;j++)
 8          prime[j]=-1;
 9       int num=0;
10       for(int i=2; i<N; i++)
11       {
12         if(prime[i]==-1)
13          {
14             num++;
15             for(int j=i;j<N;j+=i) 
16                prime[j]=num;
17          }
18       }
19       int n;   
20    while(scanf("%d",&n)!=EOF)
21        { 
22            if(n==1)
23            cout<<"0"<<endl;
24            else 
25           cout<<prime[n]<<endl;
26        } 
27    return 0;
28  }